1. Field of the Invention
The present invention relates generally to an injection mold, and more particularly, to an injection mold suitably used for molding a flat plate-shaped product having a large project area such as a substrate for an optical disc.
2. Description of the Related Art
As is generally known, in an injection molding of various resins, it is necessary to cool and solidify resins, which are melted in a heating cylinder and injected into a cavity, in the cavity and to take out the resins from the cavity. For this purpose, cooling channels for cooling and solidifying the resins in the cavity are conventionally formed in a fixed mold and a movable mold, respectively.
FIGS. 4 and 5 show the molds among the conventional injection molds for molding a compact disc (a substrate for an optical disc).
FIG. 4 shows an arrangement of cooling channels 3 viewed from a mirror surface 2 defining a cavity of a fixed mold 1, and FIG. 5 shows an arrangement of cooling channels 6 viewed from a mirror surface 5 of a movable mold 4.
In these figures, the cooling channels 3 and 6 are formed of a plurality of cooling channels (three cooling channels in these figures) 7, 8, and 9 extending to circumferential directions and the cooling channels radially communicating these cooling channels 7, 8, and 9. The cooling channels 7, 8, and 9 extending to circumferential directions are formed at equal intervals in the radial directions from the respective centers. Accordingly, when the movable mold 4 is superimposed on the fixed mold 1, these cooling channels 7, 8, and 9 formed in both fixed and movable molds 1 and 4 are arranged so that they completely correspond to each other in the radial directions of the cavity.
In the conventional injection mold as described above, the cooling channels 7, 8, and 9 of the movable mold 4 and the fixed mold 1 are arranged so that they completely correspond to each other in a radial directions of the cavity. Thus, when a compact disc is molded by these fixed and movable molds 1 and 4, a cooling rate at the positions where the cooling channels 7, 8, and 9 are not formed is inferior to that at the positions where the cooling channels 7, 8, and 9 are formed.
Thus, the conventional injection mold has the following problems.
When a cooling time is shortened at a request of increasing a cycle of injection molding, optical properties such as birefringence of the obtained optical disc becomes uneven due to a difference of the cooling rate of the resins in the cavity 2 between the sites where the cooling channels 7, 8, and 9 are formed and those where these cooling channels are not formed, or mechanical properties of the obtained optical disc is deteriorated due to occurrence of a warpage or the like.
The problems as described above will be specifically described. Firstly, using the conventional injection mold, an injection molding was carried out to obtain a compact disc having a diameter of 120 mm under the conditions of mold temperature of 80.degree. C., resin temperature of 320.degree. C. and a cooling time of 2.5 seconds. In each of the circumferential directions of seven radial directions of the obtained disc, birefringence at fifty points was measured, respectively. The average values per each of the radiuses were found from the measured values, and the average values were subtracted from each of the measured values. FIG. 6 is a circle graph showing the remainders obtained by subtracing the average values from each of the measured values. In this figure, the speckled portions show unevenness of the birefringence. The larger area of the speckled portion means the larger unevenness of the birefringence. Accordingly, as apparently from the figure, in the disc molded the conventional injection mold has a large unevenness of the birefringence in the faces thereof, thereby causing remarkably unevenness of the optical properties of the disc.